1. Field of the Invention
The present invention relates to multiphase fluid flow, and more particularly, the measurement of gas fraction, water fraction and oil fraction in such systems wherein homogeneous flow of the fluid stream being measured is not obtainable. More particularly, the invention relates to a multiphase flow measuring system capable of selectively sampling fluid flow in a multiphase flow regime at different known points across the diameter of the flow line containing the fluid being measured. Also the invention relates to a sample probe which allows fluid sampling at different known points along the probe length. Also the invention relates to correction of pressure drop flow rate instrument for inhomogeneous flow.
2. The Prior Art
Recent advances in the measurement of multiphase fluid flow are typified in co-assigned U.S. Pat. No. 5,597,961. The Texaco STARCUT(copyright) watercut monitor used in such systems irradiates fluid flow samples with microwave energy and measures the reflected, transmitted, phase and intensity of the sample to derive quantitative measures of gas fraction, water fraction and hydrocarbon fraction in the flowing fluid stream. This type of system has proven to be very accurate in use in the oil industry throughout the world.
In most non-homogeneous flow regime cases it has been found that certain components of the total volumetric flow in a line are unevenly distributed across the diameter of a perpendicular cross section of the line as the fluid flows. Knowledge this uneven distribution is essential for accurate functioning of multiphase meters. For example density contract and distribution has been shown to be of major importance in orifice meters. The STARCUT(copyright) monitor is usually placed in a sidestream flowline in the manner depicted in the previously referenced patent. If the physical sampling point is at the outer diameter or the inner diameter of the flow line being measured, an inaccurate measurement could result in an inhomogeneous multiphase fluid flow regime. There have been attempts in the prior art to make selective, radial samples in measuring this type of flow regime.
In co-assigned U.S. Pat. No. 4,977,915 the effectiveness of a demulsifier injection rate is evaluated using two sample points in a flowing multiphase stream caused to go around a bend to develop a density gradient along its radius. The STARCUT(copyright) monitor samples the flowing multiphase stream at two different radial positions along a cross section of the flow to measure the density gradient so generated and therefore appraise the effectiveness of the demulsifier treatment.
In co-assigned U.S. Pat. No. 5,234,012 a multiphase flow stream is divided in two and a STARCUT(copyright) watercut monitor is used to measure separately the flow in each stream, being installed in each branch of the divided flow lines as a sidestream monitor.
While such attempts to monitor non-homogeneous multiphase flow regimes have been successful to various degrees, none has been completely satisfactory.
There is a need to improve side stream sampling in a system using the STARCUT(copyright) watercut monitor to allow fluid composition determination at several radially dispersed locations across the diameter of a cross section of the flow line. The present invention provides such a system. A sampling probe is provided for drawing fluid samples at different radial locations across a diameter or chord of a cross section of the flow line. The entry port (inlet) of the sidestream sample probe is formed by the intersection of two slots, one a stationary longitudinally disposed slot, and the second a helically shaped slot cut in a cylinder rotating about the central axis of the stationary probe having the diametrically disposed slot. The slot may be shaped to provide a sample port shape. For example the longitudinal slot sides may be parallel or non-parallel. The helical slot may also be of non-parallel or parallel sides. The intersection hole size at any point along the sample length is therefore controlled by the slot shapes. As the rotating cylinder turns, the intersection of the two slots moves across the diameter or chord of the plane of the cross section of the flow line in which the diametrically disposed slot line is located. This provides complete sampling of the flow from one edge of the flow line (or pipe""s inner diameter or chord) to the opposite end. As the STARCUT(copyright) watercut monitor is capable of making multiple (more than 300) measurements per second, an improved measurement of the true gas, water and oil fractions of fluid in the multiphase line, even in a inhomogeneous flow regime is provided.
The rotation can be motor driven or flow driven. In general the location of the moving slot intersection is required to interpret the sample data in terms of fluid distribution in the pipeline. The intersection location is typically determined by use of conventional auto correlation methods, that is by using the inherent patterning in the sample signal. This pattern is referenced to the wall by using one of several methods: 1.) use of a switch on the rotating component to indicate start at one wall, 2.) by use of a model which extends pattern shape relative to wall.
Slot hole size and shape is important for sampling rate control. For example, the hole size may be sized to give a hole which is larger near the walls of the pipeline then in the center. In vertical flow axial symmetry of fluid properties may be assumed to be statistically valid. Fluid samples in vertical flow, for example, at the walls statistically represent volume of pipeline liquid which is greater than fluid samples at the center. In a preferred embodiment discussed herein the sample hole area is to be proportional to the distance from pipeline center. This size relation to pipe portion will give the proportional sampling value. Also the fluid velocity entering the sample probe must be greater than sample hole velocity to insure fluids stuck in the probe in the order in which they exist in the pipe. Fluid samples within the probe line to the fluid sampler detector will also blend together some what. Statistical averaging and short probe line will minimize sample blending.
The invention is best understood by reference to the following detailed description thereof, when taken in conjunction with the accompanying drawings. The drawings are intended as illustrative only and not as limitative.